When powered on, most integrated circuits comprising logic circuits, registers and flip-flops need to be reset to insure that their internal nodes do not have indeterminate logic states. This reset is done by a POR circuit, which delivers a signal RESET when the supply voltage is between two thresholds V1 and V2.
A POR circuit intervenes upon power-up (rise in the supply voltage) to deliver the signal RESET when the supply voltage reaches the threshold V1 and releases the signal RESET when the supply voltage reaches the threshold V2. The active value of the signal RESET may be 1 or 0 and the release of the signal RESET may therefore correspond to it being set to 0 or to 1.
A POR circuit also intervenes upon power-down (drop in the supply voltage) to deliver the signal RESET when the supply voltage becomes lower than the threshold V2. In fact, it is important to reset the logic circuits of an integrated circuit when the supply voltage drops below a determined minimum threshold, below which the proper operation of the integrated circuit is not insured. Below this threshold, certain elements may prove to be unstable or have indeterminate logic states. Certain operations can also be marred with errors, erasing or programming memory cells, for example.
Therefore, it is preferable to reset the integrated circuit to zero. The threshold V2 of the POR circuit is chosen to correspond to this minimum security threshold. Thus, the signal RESET is delivered each time the supply voltage goes below the threshold V2, whether it is when the integrated circuit is switched off or upon unintentional power-down.
When designing a POR circuit, the threshold V2 is generally chosen according to the characteristics of the application in which the integrated circuit is intended to be implemented. These characteristics are established by the user. For example, certain users may want the signal RESET to be sent when the voltage Vcc drops below a threshold V2 on the order of 2.5V while other users may want the signal RESET to be sent when the voltage Vcc drops below a threshold V2 on the order of 1.5 V. This requirement depends on the constraints imposed by the application. It can occur, for example, that the application comprises other integrated circuits that communicate with the integrated circuit concerned, and are likely to send invalid commands below a determined threshold V2. This is why it is preferable to reset the integrated circuit to zero below the threshold V2, even if the latter is capable of supporting lower supply voltages without malfunctioning.
The need to change the threshold V2 for sending the signal RESET according to the intended application leads to a diversification of the POR circuits and a corresponding diversification of the integrated circuits. Integrated circuits are often provided that can operate in a wide range of supply voltages, such as from 1.8 V to 5.5 V for example, but they need to be manufactured in two different versions. Each version includes a specific POR circuit having a threshold V2 compatible with the intended application.